[Windows/ Linux] Deciding upon languages/ tools/ libs/

Hi,
Im trying to code a circuit simulator. I need to draw symbols of resistors, capacitors etc on screen.

I want the software to be available on linux and windows both.

I am looking at Win32 with GDI as a possible option, but it only works with Windows. I do not need too abstract a library. I want to code at the very basic level possible. For this reason, I was planning to code the app in Win32, not using MFC or anything, until I felt the need to make it available in linux as well.

Also, I have seen applications whose source is available for windows as well as linux. Is it possible to code once and have the application available on multiplatforms?

I'm guessing, do I need to compile the same source using gcc for different platforms?

Also, in which languages/ platforms are most professional Windows applications written in? (Please exclude Java)